1. Field of the Invention
The present invention relates to a semiconductor laser module, and more specifically relates to a semiconductor laser module, which outputs an exciting laser beam of a specific wavelength in a stabilized state.
2. Prior Art
A wavelength division multiplexing (WDM) communication system has been developed as an optical communication system, which transmits signal rays of plurality. In this system, for example an Er doped optical fiber amplifier (EDFA) is placed in a predetermined position of an optical path, and a plurality of laser modules in each of which a semiconductor laser device has been incorporated as a light source are connected to the EDFA, and exciting laser beams from these laser modules are caused to enter said EDFA. Then, the signal rays transmitted from the light source of the signal rays are optically amplified, and the amplified signal rays are transmitted on the downstream side.
A laser module that outputs exciting laser beams has such a structure as shown in FIG. 1. That is, thermo-modules 2 composed of plural Peltier devices are disposed in a package 1. On the thermo-modules 2 are fixedly disposed a substrate 6 to which a laser device 3, a thermistor 4 and a lens 5, which is optical coupling means, is fixed, and in a through hole 1b formed in a side wall 1a of the package 1 is fixedly disposed an optical fiber 7 formed with a fiber grating 7a that functions as an optical feedback portion.
In this laser module, an oscillation laser beam emitted from an emitting end face (front facet) of the laser device 3 is focused on the lens 5 and enters a facet 7b of an optical fiber 7. And among the laser beams that entered the optical fiber 7, only a laser beam of a specific wavelength positioned in the vicinity of the central wavelength of a reflection bandwidth of the fiber grating 7a is fed back (returned) to the laser device 3 so that the wavelength of an oscillation laser beam from the laser device 3 is fixed to the specific wavelength. As a result, an exciting laser beam of the specific wavelength is output from the laser module.
In case of the laser module shown in FIG. 1, when the laser device 3 is heated by driving current supplied to the laser device 3 to increase the temperature of the device 3, the wavelength and light intensity of the oscillation laser beam is varied so that the optical output of the oscillation laser beam from the laser module becomes instable. To prevent such a problem, the temperature of the laser device 3 is measured with the thermistor 4 and an external control circuit (not shown) is activated to control the operating current of the thermo-module 2. As a result, the laser device 3 is cooled and the stability of the wavelength of the oscillation laser beam from the laser device 3 is attained.
However, in a case where the laser device 3 incorporated into a laser module is one, for example, a GaAs-based laser device, which is oscillated in a wavelength region of 980 nm, when a light feedback portion such as the fiber grating 7a is formed, the optical output of the obtained exciting laser beam change greatly with lapse of time to exhibit sometimes an unstable state of optical output.
It is an object of the present invention to solve the above-mentioned problems in a laser module in which a laser device, which oscillates in a wavelength region of 980 nm, is incorporated and a light feedback portion such as a fiber grating is formed, and provide a semiconductor laser module, which can realize its stability by suppressing the time-dependent changes for the optical output of an exciting laser beam.
To attain the above-mentioned object, according to the present invention, there is provided a semiconductor laser module comprising: a semiconductor laser device, an optical fiber, light-coupling means, which causes a laser beam emitted from the semiconductor laser device to enter the optical fiber, and a light-feedback portion, which reflects a laser beam of a specific wavelength, wherein a reflection spectrum shape at the light-feedback portion is substantially rectangular.
Further, there is provided a semiconductor laser module comprising: a semiconductor laser device, an optical fiber, light-coupling means, which causes a laser beam emitted from the semiconductor laser device to enter the optical fiber, and a light-feedback portion, which reflects a laser beam of a specific wavelength, wherein a reflection spectrum shape at the light-feedback portion is one in which the top portion has projected and recessed portions.